42 Brachiopoda of the Cinc'm7iati Group. 



greatly with the species ; surface marked with radiating, generally 

 straight strire, and sometimes with rounded, radiating plications. 

 Hinge usually shorter than the greatest breadth of the valves ; provided 

 in the ventral or larger valve with two teeth, situated one on eacla side 

 of the mesial fissure, and fitting into corresponding sockets in the 

 other valve. Structure probably always inipunctate. 



Ventral valve with its beak more prominent than that of the dorsal, 

 often bent or twisted, but not regularly incurved ; area generall}' high, 

 sometimes extremely so, its mesial fissure always closed, in adult shells, 

 by a convex pseudo-deltidium. Hinge teeth supported within by two 

 dental plates, which converge under the area toward the beak. Scars 

 of cardinal and adductor muscles occupying about one third to one 

 half the length of the valve (between the beak and the middle) and 

 forming two elongated, oval impressions, more or less deeply excavated, 

 one on each side of a mesial ridge or septum. 



Dorsal valve generally with its beak compressed and projecting little 

 beyond the cardinal margin ; area usually very narrow or rudimentary. 

 Cardinal process large, prominent, and bifid ; either slightly convex 

 or concave on the inner side, with each division more or less grooved 

 or emarginate at the extremity of the outer side ; on each side of, and 

 connecting with this, are the well developed socket plates. At the 

 bottom of the valve the quadruple scars of adductor muscles occupy 

 about one third the length of the valve, being arranged in pairs on 

 each side of a short mesial ridge. 



This genus occupies a place in animal development between the 

 genera Strophomena and Orthis, and is supposed to include filitexta, 

 subtenta, planumbona planoconvexo, nutans, Ilallie, etc. 



It is here to be observed, that Prof. Davidson and other authors on 

 brachiopoda entirely ignore the name ITemipronites, because, as I pre- 

 sume, it was not properly defined by Pander, and substitute for it 

 Streptorhyjichus (twisted beak), proposed and defined by Prof. King, in 

 1850. The following are the characters of the genus Streptorhyiichus, 

 as redefined by Prof. Davidson : 



"Shell inequivalve, convex or concavo-convex, externally sti'iated. 

 The smaller valve is semi-circular, the larger or ventral one possessing 

 a prolonged and oftentimes bent or tAvisted beak ; hinge line rather 

 shorter than the width of the shell. The area in the larger valve is 

 triangular, with a fissure covered by a convex pseudo-delridiura. A 

 small, narrow, rudimentary area exists, likewise, in the smaller valve. 

 'No foramen is observable, but the cardinal process is at times seen 

 partially extending under the deltidium. 



" In the interior of the larger or ventral valve a strong hinge tooth 



